Multilayer material, anti-erosion and anti-abrasion coating incorporating said multilayer material

ABSTRACT

The present invention relates to a multilayer material useful as an anti-erosion and anti-abrasion coating, and to a process for producing the multilayer material at low temperatures. The multilayer material comprises a substrate covered with at least one metallic tungsten ductile layer and at least one hard layer of a solid solution of an additional element chosen from carbon or nitrogen or a mixture of carbon and nitrogen in tungsten or a tungsten alloy, the two types of layers ar alternating. The multilayer material is particularly useful as a coating for parts used in aeronautics.

This is a continuation of application Ser. No. 08/075,519, filed Jun.14, 1993, now U.S. Pat. No. 5,547,767, dated Aug. 20, 1996.

FIELD OF THE INVENTION

The present invention relates to a multilayer material more particularlyfor permitting the production of an anti-erosion and anti-abrasioncoating, as well as to the process for producing this multilayermaterial.

The erosion of materials forming mechanical parts subject to the impactof abrasive particles such as sand or dust is a well known industrialproblem, e.g. in the aeronautical field. Thus, in the case of compressorblades used in gas turbine engines for aircraft, wear by erosion to theleading edges of the blades leads to a deterioration of the performancecharacteristics of the engine. In the same way, turbine blades used inelectrical power stations are subject to external attacks by solid, hardparticles such as e.g. sand or alumina. These problems also occur inother technical fields.

The erosion rate of the materials is defined by the volume or mass oferoded material for a given mass of instant particles. This erosion ratevaries with the incidence angle of the solid particles striking thesurface of the material in question.

DESCRIPTION OF THE RELATED ART

The erosion mechanisms have been studied by various authors and inparticular by J. P. Massoud, "Behaviour relative to erosion by solidparticles of a laser-treated T6V titanium alloy", PhD materialengineering thesis, Lyon INSA, 1988. These studies have made it possibleto make a distinction between two types of behavior of materials subjectto erosion. The first is a behavior characteristic of ductile materialssuch as metals, which deteriorate as a result of the appearance ofscratches and the removal of shavings. Their erosion rate is low with asolid particle jet with normal incidence (with respect to the materialplane), but is very high at low impact angles, i.e. approximately 20° to30°. There is also the characteristic behavior of fragile materials suchas glass, ceramics and hard materials (carbides), for which the energytransfer by impact leads to the appearance of cracks producing theremoval of material in the form of chips and splinters. The erosion rateof these materials is low with a particle jet having a limited incidenceangle (20° to 30°) and high with incidence angles of approximately 50°to 90°.

These two behaviors are respectively illustrated in the attached FIGS. 1and 2 showing the erosion rate of the tested material as a function ofthe incidence angle of the eroding particles.

In addition, complimentary studies carried out by T. Foley and A. Levy,"The effect of heat treatment on the erosion behavior of steel",Proceedings of the Conference on Wear of Materials, Reston, Va., Apr.11-14, 1983, ASME, 1983, p 346, have demonstrated that themicrostructure of the material also plays an important part. Forexample, the erosion rate of steels under different microstructuralstates (e.g. XC75 steel in the form of coarse perlite, fine perlite or aglobular structure or XC20 steel in its three globular forms) isdirectly linked with the distribution of the hard, fragile and ductilephases in the alloy, whereas the hardness of these steels only variesslightly with the state of the microstructure.

In addition, the erosion also varies as a function of the shape, sizeand distribution of the precipitates covering a substrate. G. Hickey, D.Boone, A. Levy and J. Stiglich, "Erosion of conventional andultrafine-grained materials", Thin Solid Films, 118, 321, 1984, havedemonstrated that fine SiC precipitates on steel had a better erosionresistance than a coarse precipitate.

Finally, the erosion rate is dependent on a certain number of parameterslinked with the nature of the eroding agent (size and shape of theeroding particles, their hardness and their fragility), as well as thetesting conditions, i.e. the speed of the eroding particles and theirangle of incidence.

Consequently, the above studies and research show that numerousparameters can occur and influence the erosion phenomena and sometimesit is difficult to appropriately compare the behavior of differentmaterials. It is also difficult to develop equipment and processes forprotecting the surface against erosion and abrasion by erodingparticles, which have a wide size range and whose incidence angles varyenormously. However, in order to satisfy the said erosion problems,certain materials have already been proposed.

Thus, various materials such as alloys of refractory metals, carbides,nitrides or borides have been investigated and used as anti-erosioncoatings. These materials have e.g. been deposited on aircraftcompressor blades using various procedures, such as plasma spraying,chemical vapor deposition or CVD, the transferred arc process, thecathodic sputtering process and other physical or chemical depositionprocedures.

A pack cementation process is also known and this is presently used forcoating with aluminium superalloy blades operating under oxidizing,corrosive and abrasive conditions. This treatment is carried out at atemperature exceeding 1000° C. for several hours and can consequentlynot be applied to parts made from steel or alloys, whose mechanicalproperties are deteriorated by thermal treatments at high temperatures.

With the aid of solving this problem and developing deposition processesoperating at low temperatures, so as to in particular make it possibleto treat ferrous alloy materials, C. L. Yaws and G. F. Wakefield"Proceedings of the 4th International Conference on chemical vapordeposition" published by G. F. Wakefield and J. M. Blocher Jr., TheElectrochemical Society Inc., Pennington, N.J. 1973, pp 173 and 577,developed a process for depositing titanium carbonitride on stainlesssteel compressor blades. Deposition is brought about by a chemicalreaction from the gaseous phase TiCl₄ --N(CH₃)₃ --N₂ --H₂ (CVD process)at between 600° and 700° C. The carbonitride adheres to the steel as aresult of a fine nickel layer interposed between the coating and thesubstrate.

U.S. Pat. No. 3,951,612 describes the production of chromium carbide(Cr₇ C₃) anti-erosion coatings on titanium alloy or steel compressorblades coated beforehand with a nickel coating. The nickel coatingimproves both the adhesion of the carbide coating and the mechanicalproperties of the composite coating. Cr₇ C₃ deposits are made atrelatively low temperatures between 450° and 650° C. This process iscarried out by the thermal decomposition of chromium dicumene (C₉ H₁₂)₂Cr. According to the authors of this patent, this two-layer coating hasa slightly less satisfactory erosion resistance than that proposed by G.F. Wakefield, but it has a very good resistance to thermal andmechanical shocks. This is an improvement compared with the titaniumcarbonitride coating, which is more fragile and brittle and which alsoleads to a 30 to 50% fatigue resistance deterioration of the coatedparts.

Another type of multilayer coating is described in EP-A-366 289,according to which the protecting structure has ceramic layersalternating with metallic layers. The ceramic and the metal havecomplimentary erosion resistance characteristics, one being morefragile, whereas the other is more ductile and this coating isconsidered to give the substrate on which it is deposited a goodresistance to erosion and corrosion. The concentration of materials atthe interfaces is progressive in order to improve the adhesion anduniformity of the coating. The protective coating is deposited on thesubstrate by cathodic sputtering in the reactive mode for the ceramicmaterial.

According to this patent, the metal layer is of titanium and the ceramiclayer of titanium nitride. This type of coating can bring about animprovement to the erosion behavior in the case of relatively smalleroding particles and in the case of small impact angles. However, ithas been found that multilayer materials constituted by a relativelysoft metal, such as titanium, as well as a ceramic such as titaniumnitride, have a mediocre behavior during erosion tests carried out withlarge impact angles, with large particles and high velocities. Althoughthis phenomenon has not been clearly explained, it is assumed to be dueto the limited hardness of the multilayer coating resulting from thesoft metal layer leading to a significant deformation at high energiesof the ceramic material, which by its very nature is fragile.

A deposition process similar to CVD and which is called CNTD (controllednucleation thermochemical deposition) is described in U.S. Pat. Nos.4,162,345, 4,147,820, 4,040,870 and 4,153,483. These processes make itpossible to respectively produce the deposit on a substrate of hardlayers containing tungsten or molybdenum and carbon, hard layerscontaining titanium and boron, carbon or silicon and hard layerscontaining titanium and boron. These coatings have a particularly gooderosion resistance. The deposits are generally made at temperaturesbetween 650° and 1100° C. The deposited materials have a non-columnarstructure constituted by small crystallites, which give these coatingsthe sought mechanical and erosion resistance properties.

In the aviation field, compressor blades at temperatures below 450° C.are generally made from titanium alloys having a high specific strength.Use of these alloys has become generalized, although their tribologicalproperties and their erosion resistance are mediocre. In order toincrease their life, it is consequently necessary to protect them by ananti-erosion coating deposited at below 400° C. so as not to bring abouta deterioration of the mechanical properties of said titanium alloy. Theauthors of the aforementioned U.S. patents have proposed, cf. D. G. Bhatand R. A. Holzl, "Microstructural evaluation of CM 500L, a new W--Calloy coating deposited by the controlled nucleation thermochemicaldeposition process", Thin Solid Films, 95, 105, 1982, a CNDT processoperating at a relatively low temperature of 350° to 550° C. in order toobtain an anti-erosion coating called CM 500ML.

The deposited material contains two phases, namely a tungsten phase anda W₃ C phase, the structure being constituted by small, non-columnar,tungsten crystallites (a few hundred nm) and the carbide phase isdispersed in said metal matrix. The carbon content typically varies from7 to 23 atomic % (0.5 to 1.5 weight %). This material has excellentanti-erosion properties

EP-A-411 646 describes a multilayer coating which is resistant toerosion and abrasion and more particularly intended to provideprotection against the impacts of relatively large particles, having afirst tungsten layer and a second layer containing a mixture of tungstenand tungsten carbide.

Another coating resistant to abrasion and erosion is described inEP-A-328 084. This coating also has a composite structure in which asubstantially pure, intermediate tungsten layer is deposited on thesubstrate under a layer of a mixture of tungsten and tungsten carbide.

Use is often made of tungsten in protective structures due to itsspecific mechanical properties and in particular its high hardness (1100kg.mn⁻² in Vickers scale HV) associated with a high modulus ofelasticity (406 GN.m⁻²), which gives it a good resistance to erosivewear.

P. N. Dyer, D. Garg, S. Sunder, H. E. Hintermann and M. Maillat,"Wear-resistant coatings containing tungsten carbide deposited by lowtemperature CVD", 15th International Conference on MetallurgicalCoatings, San Diego, Calif., April 1988, describe a CNTD process makingit possible to deposit anti-erosion coatings at a temperature below 500°C. These coatings are constituted by a mixture of tungsten and tungstencarbide phases such as W₂ C, W₂ C+W₃ C or W₃ C. The authors have studiedand developed hard, lamellar coatings constituted by layers of tungstenand W_(x) C layers (with x varying between 2 and 3) and incorporatingsmall crystallites. The composition, microstructure, hardness and sizeof the crystallites are controlled by varying the deposition parametersand particularly the partial methyl oxide (O(CH₃)₂) pressure in themixture of reactive gases WF₆ --O(CH₃)₂ --H₂ --Ar. The hardness of thesecoatings is superior to that of the pure W₂ C and WC carbides known fromthe literature.

In "Tungsten carbide erosion resistant coating for aerospacecomponents", Chemical vapor deposition of refractory metals andceramics, Mat. Res. Soc. Symp. Proc., T. M. Besmann and B. M. Gallois,vol. 168, Materials Research Society, Pittsburg, Pa., 1990, p 213, D.Garg and P. N. Dyer describe results relating to the erosion behavior ofmultilayer tungsten-tungsten carbide coatings deposited by CNTD. Thealternation of the metal and carbide layers not only makes it possibleto have a combination of a ductile material and a hard material, butalso obtain a compensation of the internal stresses which develop in thelayers deposited by CNTD. This makes it possible to obtain thickercoatings, while limiting the risk of adhesion losses and the separationof the coatings. Thus, the thickness of the coating must be the maximumpossible in order to bring about the maximum anti-erosion coating life.Experiments have shown that the resistance of these coatings to erosionwas superior by a factor of approximately 3 to that of uncoated AM-350stainless steel substrates.

The fact that the mechanical properties of a coating with acomposition-modulated structure can be very different from those ofcomposition-homogeneous coatings has been known for several years. In1970, J. S. Koehler, Phys. Rev. B., 2, 547, 1970, proposed increasingthe mechanical strength of materials by inhibiting the mobility ofdislocations as a result of a composition-modulated structure.

The analysis of these literature data reveals the interest of developinga process allowing the low temperature deposition (below 400° C.) ofanti-erosion and anti-abrasion coatings protecting low thermal strength,alloy parts from the impacts of solid particles such as sand, silica,alumina or dust.

SUMMARY OF THE INVENTION

Therefore the invention relates to a multilayer material incorporating asubstrate covered with at least one ductile layer of metal tungsten ortungsten alloy and at least one hard layer of a solid solution of anaddition element chosen from among carbon or nitrogen, in tungsten or ina tungsten alloy, the two types of layers alternating.

Preferably, the carbon content in the solid carbon solution in thetungsten or tungsten alloy is below 25, preferably between 12 and 18 orin even more preferred manner between 14 and 15 atomic %. Preferably,the nitrogen content in the solid nitrogen solution in the tungsten ortungsten alloy is between 0.5 and 15 or preferably between 1 and 10atomic %. These materials are stable, microcrystalline and hard.

The crystal lattice of the solid solution of the addition element in thetungsten corresponds to that of metallic tungsten, which is verythermally stable, because the melting point of the tungsten is close to3400° C.

This hard, ductile structure of the multilayer material according to theinvention makes it possible to improve the erosion behavior with respectto a wide range of particle sizes. The high hardness is of particularinterest at low impact angles of the eroding particles, while a goodductility makes it possible to delay the cracking of the coating at highenergy levels and for large particle sizes.

The Vickers hardness of the hard carbon layer in the tungsten reaches26,000 MPa for a charge of 50 g, when the carbon content in the solidsolution is between 14 and 15 atomic %. For comparison purposes, theVickers hardness of the ductile tungsten layer under the same conditionsis approximately 13,000 MPa.

The materials constituted by a stack having a two-layer tungsten/solidcarbon solution in tungsten structure have performance characteristicssuperior to those of coatings constituted solely by the WC_(1-x) carbidecarbon solution in tungsten structure have performance characteristicsphase. This is possibly due to the fact that the two layers, althoughhaving very different hardness levels, have an identical crystallattice, which gives a high stability to the metal/carbon phaseinterface.

The hardness of the solid nitrogen solution in the tungsten is afunction of the incorporated metalloid level. For example, the Vickershardness of the solid solution incorporating 6 atomic % nitrogen exceeds3500 kgf.nm⁻², i.e. 34,000 MPa under a charge of 100 g (HV hardness0.1).

In the multilayer material, the substrate is preferably a titanium alloyor stainless steel. However, it would also be possible to choose thematerial of the substrate from among the following materials: aluminumalloys, nickel alloys, polymers and composite materials. Thus, thedeposits can be made on materials presently used, particularly in theaeronautical industry.

The invention also relates to an anti-erosion and anti-abrasion coatingincorporating the aforementioned multilayer material.

Finally, the invention also relates to a process for the production ofthe aforementioned multilayer material comprising introducing asubstrate into a cathodic sputtering or evaporation enclosure andperforming a cathodic sputtering or evaporation respectively from atarget or a source of pure tungsten or tungsten alloy in a plasmaalternatively constituted by a rare gas or a mixture of a rare gas andan addition gas, the latter being chosen from among a gas containingcarbon and/or nitrogen so as to alternatively deposit on the substrateat least one ductile metallic tungsten or tungsten alloy layer and atleast one hard layer of a solid solution of an addition element intungsten or one of its alloys.

This process makes it possible to make deposits at low temperatures onsubstrates constituted by alloys or not very thermally stable materialssuch as aluminum alloys, polymers or composite materials. Thus, it hasbeen found during tests that the temperature of the substrate only roseto 250° to 270° C. during deposition.

This process makes it possible to only use a single equipment for thedeposition of multilayer coatings without having to vent the substratesbetween the deposition operations of the different layers, thusprotecting the interfaces against oxidation. Moreover, this processmakes it possible to modify the composition of the coating by merelymodifying the composition of the plasma within the sputtering enclosure.

This process also makes it possible to carry out the alternation of thelayers of materials with a process operating in vacuo, only using theelements to be incorporated into the layers, i.e. the tungsten and theaddition element and in the absence of other chemical elements, such ase.g. fluorine, present in the CNTD process and which may contaminate thecoating. This contamination more particularly can occur at theinterfaces during the modification of the deposition parameters frompassing from the deposition of one layer to another. This contaminationof the interfaces is prejudicial in that foreign elements are liable tobring about a deterioration of the mechanical properties of the layersand may lead to the propagation or initiation of cracks or thedeterioration of the corrosion strength.

Finally, this process makes it possible to automate the substratecharging and discharging operations, as well as the depositionoperations.

Advantageously, in the process described hereinbefore, the source of thepure tungsten or one of its alloys used in cathodic sputtering is amagnetron-type target. This makes it possible to deposit the coating ata speed exceeding 2.5 nm/s and obtain thick coatings with a reasonabledeposition time. Moreover, these coatings adhere very well to thesubstrate as a result of the particles ejected from the target reachingthe substrate at a kinetic energy of several eV, which corresponds to arelatively high energy compared with that used in other depositionprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail hereinafter relative tonon-limitative embodiments and with reference to the attached drawings,wherein show:

FIGS. 1 and 2 Curves illustrating the relationship between the erosionrate and the incidence angle of eroding particles, respectively for aductile material of the metallic type and for a fragile material of thecarbide type.

FIGS. 3 to 6 Diagrammatic sections of different embodiments of themultilayer material according to the invention.

FIG. 7 Two lines C₁ and C₂ illustrating the respective carbon andtungsten composition of a hard carbon-tungsten layer of the materialaccording to the invention, as a function of the methane flow rate usedin the plasma during deposition.

FIG. 8 The variation of the hardness of a hard layer of solid solutionof carbon in tungsten of the coating according to the invention, as afunction of the carbon content.

FIG. 9 A diagram illustrating the erosion rate of the two examples ofmultilayer material obtained according to the invention compared withcontrols, the erosion being performed with eroding particles reachingthe surface of said material with an incidence angle of 90°.

FIG. 10 Similar to FIG. 9, except with respect to the incidence anglewhich is 45°.

FIG. 11A diagram similar to FIG. 9, but obtained under slightlydifferent erosion conditions.

FIG. 12 Similar to FIG. 11, except that the incidence angle is 30°.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated in FIG. 3, the multilayer material according to theinvention comprises a substrate 2 covered with alternating ductiletungsten or tungsten alloy layers 4 and hard layers 6 of a solidsolution of an addition element in tungsten or in a tungsten alloy. Thisalternation of ductile layers and hard layers forms a coating 8. Theaddition element is chosen from among carbon or nitrogen.

The tungsten alloy usable in the ductile layer 4 or in the hard layer 6is an alloy constituted by tungsten and one or more other elementschosen from among titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, nickel, copper, aluminum, cobalt, iron,platinum and palladium.

As stated hereinbefore, the carbon content in the solid carbon solution6 in tungsten or a tungsten alloy is below 25, preferably between 12 and18 or even more preferably between 14 and 15 atomic %. The nitrogencontent of the solid nitrogen solution 6 in the tungsten or tungstenalloy is between 0.5 and 15 and preferably between 1 and 10 atomic %.

FIG. 4 illustrates a second embodiment of the multilayer materialaccording to the invention. In this case, it successively comprises inalternating manner a ductile tungsten or tungsten alloy layer 4 and atleast two hard layers of a solid solution of an addition element intungsten or one of its alloys. Three hard layers 6, 6a, 6b are shown andwhich have constant addition element contents within each respectivelayer, but which increase in the totality of the three successive hardlayers. In this case, the content increases between the substrate 2 andthe exterior of the coating. In other words, the hard layer 6 closest tothe substrate 2 has the lowest addition element content and the layer 6bthe highest content.

It should be noted that the superimposing order of the layers could bereversed, the layer 6b then being placed in the vicinity of thesubstrate 2 and the layer 6 to the exterior of the coating. The choiceof the order of the layers is a function of the envisaged application.

FIG. 5 illustrates a variant similar to FIG. 4, in which at least one ofthe hard layers is internally provided with an increasing gradient ofaddition element in the solid solution, the part of said hard layerhaving the lowest addition element content being directed towards thesubstrate 2 (or optionally vice versa). The hard layer with the risinggradient is 6c.

Finally, FIG. 6 illustrates a fourth embodiment of the multilayermaterial according to the invention. In this case, the substrate 2 iscoated with a first tungsten or tungsten alloy layer 4, on which aresuccessively deposited three solid solution layers 6a, 6b, 6a, theaddition element content being constant within each hard layer, but twosuccessive hard layers have different addition element contents. Thus,the addition element content of the intermediate layer 6b is higher thanthat of the two layers 6a on either side. Finally, on said assembly isdeposited a second tungsten or tungsten alloy layer 4, which isoptionally coated with other layers 6a and 6b. In exemplified manner,substrates have been produced having a tungsten layer covered with threelayers of solid nitrogen solutions in tungsten, in which the nitrogencontents were respectively 3, 6 and 3 atomic %.

Preferably, although this is not a limitative feature of the invention,the ductile tungsten or tungsten alloy layer 4 is placed directly incontact with the substrate 2, whilst the outer layer of the coating is asolid solution layer of an addition element in tungsten or one of itsalloys.

The number of layers, the thickness of each layer and the totalthickness of the thus formed anti-erosion and anti-abrasion coating 8,as well as the addition element concentration of the hard solid solutionlayers 6 are chosen as a function of the sought degree of protectionagainst abrasion and erosion and as a function of the envisagedapplication.

The total thickness of the anti-erosion and anti-abrasion coatingapplied to the substrate can be between 5 and 200 μm and preferablybetween 10 and 100 μm. The number of layers can be between 2 and 500 andthe thickness of each ductile layer 4 and hard layer 6 can be between0.01 and 50 μm, preferably between 0.05 and 25 μm or in even morepreferred manner between 0.2 and 15 μm. For example, in the standarderosion protection applications, the total thickness is generallybetween 40 and 80 μm.

The successive deposition of the ductile tungsten layers 4 and the hardlayers 6 is carried out on a substrate 2, e.g. polished with diamondpaste. Deposition preferably takes place under a reduced pressure, e.g.in a low pressure enclosure equipped with a known device for bringingabout the vapor phase, such as a cathodic sputtering or electron beambombardment system.

This apparatus can operate with or without a magnetic field, in thediode or triode mode, being supplied with alternating or direct current,combined with an electric field established between the substrate 2 andthe tungsten or tungsten alloy source, under plasma-producing pressureconditions. In the case of a deposition by cathodic sputtering, use isgenerally made of a planar pure tungsten or tungsten alloy target,preferably of the magnetron type.

This deposition by evaporation or cathodic sputtering takes place in aplasma alternatively constituted by a rare gas (e.g. argon) or a mixtureof a rare gas and an addition gas, the latter being chosen from among agas containing carbon or nitrogen, as a function of whether it is wishedto obtain the deposition of a solid solution respectively of carbon ornitrogen in tungsten or one of its alloys. The gas containing the carbonis preferably a hydrocarbon and in particular methane. It also fallswithin the scope of the Expert to introduce an addition gassimultaneously containing carbon and nitrogen, in order to deposit amixed solid solution containing carbon and nitrogen as additionelements.

Working takes place under a rare gas pressure preferably between 0.1 and10 Pa (e.g. 0.3 Pa) and a partial nitrogen or methane pressurepreferably between 0.01 and 1 Pa, as a function of the nature of thedeposit which it is wished to obtain. This pressure is determined as afunction of the power density applied to the deposition source and thedistance between the source and the substrate.

Taking account of the volume of the enclosure and the size of thecathodic sputtering or evaporation source the various settings such asthe distance between the substrate 2 and the source are carried out inaccordance with conventional procedures. It is necessary to take accountof the partial pressure of the gas used for a power density applied tothe source and check by crystallographic analysis the product obtained,so as to avoid the formation of a carbide or nitride phase.

When it is wished to form a multilayer material with a solid carbonsolution in tungsten, the methane content is varied between 0% fordepositing a tungsten or tungsten alloy layer 4 and values exceeding 5%and below 25% by volume for depositing the following hard layer 6. Intests performed, the argon flow rate was 50 cm³ /min and the methanerate varied between 4 and 15 cm³ /min. The introduction of methane intothe sputtering enclosure in order to deposit the hard layer 6 leads toan increase in the total pressure, which then rises to 6.10⁻¹ Pa, whenthe argon-methane plasma contains 50% methane.

Thus, contrary to the prior art where deposits were made in the presenceof argon and acetylene, it has been found that the use of methane madeit possible to obtain very low carbon contents in the tungsten.

When in the tests performed the aim was to form a multilayer materialwith a solid nitrogen solution in tungsten, the nitrogen content in theplasma was varied between 0% for depositing a tungsten or tungsten alloylayer 4 and values exceeding 9% and below 30% for depositing thefollowing hard layer 6. In these tests, the nitrogen flow rate variedbetween 10 and 40 cm³ /min and the argon rate was close to 100 cm³ /min.

The average sputtering power density is preferably between 6 and 12W/cm² for a magnetron target and the substrate 2 is polarized at -100 V.The substrate 2 is not deliberately heated, but it is found that itstemperature which was approximately 150° C. during the surface treatmentrises to 250° to 270° C. during the deposition stage.

Certain more specific measurements were carried out during thedeposition of a solid carbon solution in tungsten. As illustrated inFIG. 7, the carbon content (curve C₁) in the layer 6 deposited on thesubstrate increases linearly when the methane flow introduced into theplasma increases. Conversely, th tungsten content (curve C₂) decreases.By varying the methane content in the argon-methane plasma, it is thuspossible to obtain two tungsten-carbon material types. When the methanecontent is below 25% in the argon-methane plasma, the carbon content inthe hard layer 6 is below 25 atomic %. However, when the methane contentin the plasma is between 25 and 50%, the carbon content of thetungsten-carbon material passes from 30 to 65 atomic %.

However, only the first material obtained containing less than 25 atomic% carbon is of interest in the invention. The crystallographic structureof the first material obtained measured by X-ray diffraction correspondsto that of α phase metallic tungsten. However, the cubic latticespacing, calculated from diffraction spectra, is higher than that of themetallic tungsten lattice supplied in the literature. The expansion ofthe lattice spacing increases to 3% when the carbon content in the layerreaches 15 atomic %. This is why it is possible to accept that the layer6 containing less than 25 atomic % carbon is a solid carbon solution intungsten. The expansion of the lattice spacing may also be due to theexistence of internal mechanical stresses. The expansion of the crystallattice of the metal can be due to a combination of these two effects.

The Vickers hardness of the hard layer 6 increases progressively withthe carbon content, as illustrated in FIG. 8. Thus, under a load of 50g, the said hardness reaches a peak of 26,000 MPa for a solid carbonsolution in tungsten containing approximately 15 atomic % carbon. Beyond15 atomic % carbon, the hardness of the coating decreases when thecarbon content increases.

For illustration purposes, a number of examples of a multilayer materialaccording to the invention will now be given, as well as the results oferosion tests performed on some of these materials.

Multilayer material with solid solutions containing carbon

The deposition of double layers 4 and 6 according to the invention wascarried out on a TA6V titanium alloy substrate using the aforementionedcathodic sputtering deposition method. The hard layers 6 were formed bya solid carbon solution in tungsten. The carbon content in the solidsolution 6 was 15 atomic %, which corresponds to the maximum hardness of26,000 MPa. The total thickness of the coating 8 was approximately 60μm.

There were two variants:

EXAMPLE 1

The thickness of the elementary layers of tungsten 4 or solid solution 6with 15 atomic % carbon was 10 μm and in all 6 layers were deposited.

EXAMPLE 2

The thickness of the elementary layers of tungsten 4 or solid solution 6with 15 atomic % carbon was 5 μm and 12 layers were deposited.

EROSION TEST PERFORMED ON EXAMPLES 1 AND 2, on TA6V AND ON A COMPARATIVEEXAMPLE 1 COMPARATIVE EXAMPLE 1

A comparative example was obtained by carrying out the successivedeposition of pure tungsten and tungsten carbide layers. The thicknessof the elementary tungsten and tungsten carbide layers with 40 atomic %carbide was 5 μm and in all 12 layers were deposited. The substrate wasof TA6V titanium alloy.

The multilayer materials produced in accordance with the threeaforementioned examples and TA6V underwent erosion tests, whose resultsare given in FIGS. 9 and 10. The erosion rate values are expressed bythe ratio between the coating mass lost by erosion and the erodingparticle mass used in the test. The lower the erosion rate the betterthe tested sample. These erosion tests were carried out in accordancewith two incidence angles of the eroding particles, respectively 90° and45° and represented respectively in FIGS. 9 and 10. The eroding agentwas quartz with a grain size of 160 μm, the eroding agent flow rate was6 g/min and the carrier air velocity was 240 m/s.

The erosion rate of the uncoated TA6V alloy substrate is equal to orhigher than 10⁻³ g/g for incidence angles of 90° and 45°. This alsoapplies with respect to the erosion rate of the tungsten/tungstencarbide material of comparative example 1.

As can be seen from FIGS. 9 and 10, the multilayer coating producedaccording to example 1 makes it possible to improve the erosionresistance by a factor of 10 or 100 compared with TA6V alone, dependingon whether the incidence angle of the eroding particles is 90° or 45°.The multilayer coating corresponding to example 2 leads to animprovement of the erosion resistance by a factor of 100 orapproximately 200 depending on whether the incidence angle of theparticles is 90° or 45°. Therefore the second coating is better than thefirst and has performance characteristics greatly superior to those ofthe tungsten/tungsten carbide coatings of comparative example 1.

Multilayer material with solid solutions containing nitrogen

EXAMPLE 3

Use is made of a vacuum deposition equipment equipped with a planarmagnetron, cathodic sputtering source, having a tungsten target ofpurity 99.5. The steel or titanium alloy substrates connected to thenegative pole of a direct current generator are positioned facing thesource at a distance of 9 cm.

After obtaining a pressure below 8×10⁻⁴ Pa in the deposition enclosure,argon is introduced up to a pressure of 0.3 Pa. This is followed bycleaning of the substrate by cathodic sputtering under a voltage of -200V, for 30 min and the deposition source with a power density of 12W.cm⁻², for 10 min.

Following these two operations, the substrate is coated. The biasvoltage of the substrate is reduced to -100V and the power densityapplied to the source remains unchanged. Under these conditions, atungsten layer with a thickness of 2 μm is obtained in 6 minutes, i.e. adeposition rate of 5.5 nm.s⁻¹.

When the desired tungsten layer thickness is reached, sequentialintroduction takes place of a nitrogen flow until a partial pressure of0.045 Pa is obtained, which corresponds to a nitrogen flow of 50standard cm³ /minutes⁻¹ (50 sccm), the partial argon pressure is 0.3 Paand the power density applied to the source of 12 W.cm⁻² remainingunchanged. These parameters lead to the formation of a tungsten-nitrogensolid solution layer 2 μm thick within 8 minutes, i.e. a deposition rateof 4.2 nm.s⁻¹.

The nitrogen flow is then stopped and the sequence repeated 14 timesuntil a multilayer coating with a total thickness of 56 μm is obtained.

EXAMPLE 4

The titanium alloy substrate (TA6V) and the sputtering source arecleaned, working taking place at a partial argon pressure of 0.2 Pa anda power density to the source of 12 W.cm⁻².

The first pure tungsten layer with a thickness of 12 μm is deposited ina pure argon atmosphere at a speed of 5.5 nm.s⁻¹.

The second, 12 μm thick, solid nitrogen solution layer is deposited inthe presence of a partial nitrogen pressure of 0.06 Pa, i.e. a nitrogenflow of 10 sccm, at a speed of 5.4 nm.s⁻¹.

The third, 12 μm thick, solid nitrogen solution layer is deposited witha nitrogen flow of 20 sccm, corresponding to a partial nitrogen pressureof 0.016 Pa, at a speed of 5 nm.s⁻¹.

The fourth, 12 μm thick, solid nitrogen solution layer is deposited witha nitrogen flow of 30 sccm, corresponding to a partial nitrogen pressureof 0.025 Pa, at a speed of 4.7 nm.s⁻¹.

The fifth, 12 μm thick solid nitrogen solution layer is deposited with anitrogen flow of 40 sccm, corresponding to a partial nitrogen pressureof 0.035 Pa, at a speed of 4.4 nm.s⁻¹.

With such a deposition sequence a coating with a total thickness of 60μm is obtained having a composition gradient and therefore incrementedmechanical properties.

EXAMPLE 5

A coating identical to that of example 4 was produced, except that a 0.5μm thick tungsten layer was interposed between each nitrogen-tungstensolid solution layer.

EXAMPLE 6

After cleaning the substrate and the sputtering source, working tookplace with a partial argon pressure of 0.3 Pa and a source power densityof 12 W.cm⁻².

The first, 10 μm thick tungsten layer was deposited in a pure argonatmosphere at a speed of 5.5 nm.s⁻¹.

The second, 5 μm thick solid solution layer was deposited in thepresence of a partial nitrogen pressure of 0.016 Pa, i.e. a nitrogenflow of 20 sccm, at a speed of 5 nm.s⁻¹.

The third, 10 μm thick, solid solution layer was deposited with anitrogen flow of 50 sccm, corresponding to a partial nitrogen pressureof 0.045 Pa, at a speed of 4.2 nm.s⁻¹.

The fourth, 5 μm thick, solid solution layer was deposited in thepresence of a partial nitrogen pressure of 0.016 Pa, i.e. a nitrogenflow of 20 sccm, at a speed of 5 nm.s⁻¹.

This sequence was repeated twice to obtain a multilayer coating 60 μmthick.

EXAMPLE 7

Use was made of a deposition equipment equipped with an electron beamevaporation source with an electric power of 15 kW. The evaporationcrucible, with a capacity of 60 cm³, was filled with diameter 20 mm, 5mm thick tungsten pellets. The substrates were placed above theevaporation source and at a distance of 30 cm.

After placing the deposition enclosure under a vacuum and cleaning thesubstrates by ion bombardment, the actual deposition phase was carriedout. The bias voltage applied to the substrates was reduced to -100 Vand the argon pressure to 0.2 Pa. Either manually or by means of anautomatic device, the power of the electron beam was regularly raised to10 kW, so as to progressively bring about the melting of the tungstenand then the evaporation.

The first, 2 μm thick pure tungsten layer was deposited at a speed of 6nm.s⁻¹

The second, 4 μm thick layer was produced in the reactive mode in thepresence of a nitrogen low of 40 sccm. The partial nitrogen pressure wasmaintained at a value of 0.04±0.001 Pa, by adjusting the power of theelectron beam.

The sequence was repeated 5 times to obtain a 30 μm thick coating.

EXAMPLE 8

The procedure of example 6 was used, but having as the source atungsten-chromium, with 3 atomic % chromium, alloy and working at apartial argon pressure of 0.4 Pa and a power density of 11 W.cm⁻².

The first, 10 μm thick pure tungsten layer was deposited in a pure argonatmosphere at a speed of 5.6 nm.s⁻¹.

The second, third and fourth layers were deposited in the mannerdescribed in example 6, hut the thickness of the fourth layer was 10 μm.The sequence was repeated twice to obtain a 70 μm multilayer coating.

EROSION TESTS CARRIED OUT ON EXAMPLES 2 AND 5, TA6V AND COMPARATIVEEXAMPLE 2 COMPARATIVE EXAMPLE 2

The titanium alloy substrate (TA6V) was covered with a 60 μm thick, TiNmonolayer. The uncoated TA6V alloy and the materials produced accordingto examples 2 and 5 and comparative example 2 than underwent erosiontests, whose results are given in FIGS. 11 and 12. These erosion testswere carried out under the same conditions as the erosion test describedhereinbefore, except that in this case the eroding agent was quartzhaving grains of 600 μm and the eroding agent flow rate was 2 g/min.These erosion tests were performed in accordance with two incidenceangles of the eroding particles, respectively 90° and 30° and shownrespectively in FIGS. 11 and 12.

As can be seen from FIG. 11, the multilayer coating produced accordingto examples 2 and 5 makes it possible to improve the erosion resistanceby a factor of approximately 200 compared with TA6V and approximately300 compared with TiN.

As can be gathered from FIG. 12, the multilayer coating according toexamples 2 and 5 improves the erosion resistance by a factor ofapproximately 150 compared with TA6V and is identical to TiN.

We claim:
 1. A multilayer material, characterized in that it comprises asubstrate covered with at least one ductile, metallic tungsten ortungsten alloy layer and at least one hard layer of a solid solution ofnitrogen, in tungsten or in a tungsten alloy, the two types of layersalternating.
 2. Multilayer material according to claim 1, characterizedin that the nitrogen content in the solid solution is between 0.5 and 15atomic %.
 3. Multilayer material according to claim 1, characterized inthat the nitrogen content in the solid solution is between 1 and 10atomic %.
 4. Multilayer material according to claim 1, characterized inthat it comprises at least two successive hard layers of solid solutionhaving constant nitrogen contents within each hard layer, and twosuccessive hard layers having different nitrogen contents.
 5. Multilayermaterial according to claim 1, characterized in that it comprises atleast two successive hard layers of solid solution having constantnitrogen contents within each hard layer and increasing nitrogencontents within the assembly of successive hard layers.
 6. Multilayermaterial according to claim 1, characterized in that it comprises atleast two successive hard layers of solid solution having constantnitrogen contents within each hard layer and increasing nitrogencontents within the assembly of successive hard layers, and in that thehard layer closest to the substrate has the lowest nitrogen content ofall the successive hard layers.
 7. Multilayer material according toclaim 1, characterized in that at least one of the hard layersinternally has an increasing gradient of nitrogen in the solid solution,the part of the said hard layer having the lowest nitrogen content beingdirected towards the substrate.
 8. Multilayer material according toclaim 1, characterized in that the tungsten alloy used is an alloyconstituted by tungsten and one or more other elements selected from thegroup consisting of titanium, zirconium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, nickel, copper, aluminum, cobalt, iron,platinum and palladium.
 9. Multilayer material according to claim 1,characterized in that the substrate is chosen from among a titaniumalloy, stainless steel, an aluminum alloy, a nickel alloy, polymers orcomposite materials.
 10. A mechanical part subject to the impact ofabrasive particles including an anti-erosion and anti-abrasion coating,characterized in that it comprises a multilayer material according toany one of the claims 1-9.